Gold nanoparticles and polyethylene glycols functionalized conducting polyaniline nanowires for ultrasensitive and low fouling immunosensing of alpha-fetoprotein

Recent Advances in Biosensor Technology for Early-Stage Detection of Hepatocellular Carcinoma-Specific Biomarkers: An Overview

Hepatocellular carcinoma is currently the most common malignancy of the liver. It typically occurs due to a series of oncogenic mutations that lead to aberrant cell replication. Most commonly, hepatocellular carcinoma (HCC) occurs as a result of pre-occurring liver diseases, such as hepatitis and cirrhosis. Given its aggressive nature and poor prognosis, the early screening and diagnosis of HCC are crucial. However, due to its plethora of underlying risk factors and pathophysiologies, patient presentation often varies in the early stages, with many patients presenting with few, if any, specific symptoms in the early stages. Conventionally, screening and diagnosis are performed through radiological examination, with diagnosis confirmed by biopsy. Imaging modalities tend to be limited by their requirement of large, expensive equipment; time-consuming operation; and a lack of accurate diagnosis, whereas a biopsy's invasive nature makes it unappealing for repetitive use. Recently, biosensors have gained attention for their potential to detect numerous conditions rapidly, cheaply, accurately, and without complex equipment and training. Through their sensing platforms, they aim to detect various biomarkers, such as nucleic acids, proteins, and even whole cells extracted by a liquid biopsy. Numerous biosensors have been developed that may detect HCC in its early stages. We discuss the recent updates in biosensing technology, highlighting its competitive potential compared to conventional methodology and its prospects as a tool for screening and diagnosis.

Electrochemical protein biosensors for disease marker detection: progress and opportunities

The development of artificial intelligence-enabled medical health care has created both opportunities and challenges for next-generation biosensor technology. Proteins are extensively used as biological macromolecular markers in disease diagnosis and the analysis of therapeutic effects. Electrochemical protein biosensors have achieved desirable specificity by using the specific antibody-antigen binding principle in immunology. However, the active centers of protein biomarkers are surrounded by a peptide matrix, which hinders charge transfer and results in insufficient sensor sensitivity. Therefore, electrode-modified materials and transducer devices have been designed to increase the sensitivity and improve the practical application prospects of electrochemical protein sensors. In this review, we summarize recent reports of electrochemical biosensors for protein biomarker detection. We highlight the latest research on electrochemical protein biosensors for the detection of cancer, viral infectious diseases, inflammation, and other diseases. The corresponding sensitive materials, transducer structures, and detection principles associated with such biosensors are also addressed generally. Finally, we present an outlook on the use of electrochemical protein biosensors for disease marker detection for the next few years.

Electrochemical immuno-biosensors for the detection of the tumor marker alpha-fetoprotein: A review

Alpha-fetoprotein (AFP) is a glycoprotein that has many important physiological functions, including transportation, immunosuppression, and induction of apoptosis by T lymphocytes. AFP is closely related to the development of hepatocellular carcinoma and many kinds of tumors, all of which can show high concentrations, so it is used as a positive test indicator for many kinds of tumors. This paper reviews recent advances in the detection of the tumor marker AFP based on three immuno-biosensors: electrochemical (EC), photoelectrochemical (PEC), and electrochemical luminescence (ECL). The electrodes are modified by different materials or homemade composites, different signaling molecules are selected as single probes or dual probes for the detection of AFP. The detection limit was as low as 3 fg/mL, which indicated that the AFP immunosensor had achieved highly sensitive detection. In addition, we also reviewed and summarized the current development status and application prospect of AFP immunoelectrochemical sensors. There are not too many researches on immunosensors based on dual-signal ratios, and the commonly used probes are methylene blue (MB) and ferrocene (Fc). It would be more innovative to have more novel signaling molecules as probes to prepare dual-signal ratio sensors.

Enhanced split-type photoelectrochemical aptasensor incorporating a robust antifouling coating derived from four-armed polyethylene glycol

Antifouling biosensors capable of preventing protein nonspecific adhesion in real human bodily fluids are highly sought-after for precise disease diagnosis and treatment. In this context, an enhanced split-type photoelectrochemical (PEC) aptasensor was developed incorporating a four-armed polyethylene glycol (4A-PEG) to construct a robust antifouling coating, enabling accurate and sensitive bioanalysis. The split-type PEC system involved the photoelectrode and the biocathode, effectively separating signal converter with biorecogniton events. Specifically, the TiO2 electrode underwent sequential modification with ZnIn2S4 (ZIS) and polydopamine (PDA) to form the PDA/ZIS/TiO2 photoelectrode. The cathode substrate was synthesized as a hybrid of N-doped graphene loaded with Pt nanoparticles (NG-Pt), and subsequently modified with 4A-PEG to establish a robust antifouling coating. Following the anchoring of probe DNA (pDNA) on the 4A-PEG-grafted antifouling coating, the biocathode for model target of cancer antigen 125 (CA125) was obtained. Leveraging pronounced photocurrent output of the photoelectrode and commendable antifouling characteristics of the biocathode, the split-type PEC aptasensor showcased exceptional detection performances with high sensitivity, good selectivity, antifouling ability, and potential feasibility.

Anti-Fouling Strategies of Electrochemical Sensors for Tumor Markers

The early detection and prognosis of cancers require sensitive and accurate detection methods; with developments in medicine, electrochemical biosensors have been developed that can meet these clinical needs. However, the composition of biological samples represented by serum is complex; when substances undergo non-specific adsorption to an electrode and cause fouling, the sensitivity and accuracy of the electrochemical sensor are affected. In order to reduce the effects of fouling on electrochemical sensors, a variety of anti-fouling materials and methods have been developed, and enormous progress has been made over the past few decades. Herein, the recent advances in anti-fouling materials and strategies for using electrochemical sensors for tumor markers are reviewed; we focus on new anti-fouling methods that separate the immunorecognition and signal readout platforms.

Conducting Polymers as Versatile Tools for the Electrochemical Detection of Cancer Biomarkers

The detection of cancer biomarkers has recently become an established method for the early diagnosis of cancer. The sensitive analysis of specific biomarkers can also be clinically applied for the determination of response to treatment and monitoring of disease progression. Because of the ultra-low concentration of cancer biomarkers in body fluids, diagnostic tools need to be highly sensitive and specific. Conducting polymers (CPs) are particularly known to exhibit numerous features that enable them to serve as excellent materials for the immobilization of biomolecules and the facilitation of electron transfer. Their large surface area, porosity, and the presence of functional groups provide CPs with binding sites suitable for capturing biomarkers, in addition to their sensitive and easy detection. The aim of this review is to present a comprehensive summary of the available electrochemical biosensors based on CPs and their composites for the ultrasensitive detection of selected cancer biomarkers. We have categorized the study based on different types of targeted biomarkers such as DNAs, miRNAs, proteins, enzymes, neurotransmitters and whole cancer cells. The sensitivity of their detection is enhanced by the presence of CPs, providing a limit of detection as low as 0.5 fM (for miRNA) and 10 cells (for the detection of cancer cells). The methods of multiplex biomarker detection and cell capture are indicated as the most promising category, since they furnish more accurate and reliable results. Ultimately, we discuss the available CP-based electrochemical sensors and promising approaches for facilitating cancer diagnosis and treatment.

Hybrid Nanobioengineered Nanomaterial-Based Electrochemical Biosensors

Nanoengineering biosensors have become more precise and sophisticated, raising the demand for highly sensitive architectures to monitor target analytes at extremely low concentrations often required, for example, for biomedical applications. We review recent advances in functional nanomaterials, mainly based on novel organic-inorganic hybrids with enhanced electro-physicochemical properties toward fulfilling this need. In this context, this review classifies some recently engineered organic-inorganic metallic-, silicon-, carbonaceous-, and polymeric-nanomaterials and describes their structural properties and features when incorporated into biosensing systems. It further shows the latest advances in ultrasensitive electrochemical biosensors engineered from such innovative nanomaterials highlighting their advantages concerning the concomitant constituents acting alone, fulfilling the gap from other reviews in the literature. Finally, it mentioned the limitations and opportunities of hybrid nanomaterials from the point of view of current nanotechnology and future considerations for advancing their use in enhanced electrochemical platforms.

Biorecognition antifouling coatings in complex biological fluids: a review of functionalization aspects

Recent progress in biointerface research has highlighted the role of antifouling functionalizable coatings in the development of advanced biosensors for point-of-care bioanalytical and biomedical applications dealing with real-world complex samples. The resistance to nonspecific adsorption promotes the biorecognition performance and overall increases the reliability and specificity of the analysis. However, the process of modification with biorecognition elements (so-called functionalization) may influence the resulting antifouling properties. The extent of these effects concerning both functionalization procedures potentially changing the surface architecture and properties, and the physicochemical properties of anchored biorecognition elements, remains unclear and has not been summarized in the literature yet. This critical review summarizes these key functionalization aspects with respect to diverse antifouling architectures showing low or ultra-low fouling quantitative characteristics in complex biological media such as bodily fluids or raw food samples. The subsequent discussion focuses on the impact of functionalization on fouling resistance. Furthermore, this review discusses some of the drawbacks of available surface sensitive characterization methods and highlights the importance of suitable assessment of the resistance to fouling.

2D transparent few-layered hydrogen substituted graphdiyne nano-interface for unprecedented ultralow ANXA2 cancer biomarker detection

Herein, we report synthesis of 2D few-layered transparent hydrogen substituted graphdiyne (HsGDY) nanosheets and explored its electrochemical characteristics for the first time to develop a nano-interface for cancer biomarker detection [liver cancer (LC) biomarker; ANXA2]. The semiconducting HsGDY (band gap; 1.98 eV) contains considerable number of sp and sp² hybridised π-electrons with abundant hierarchical pores, thus reveals a negative peripheral charge and high surface area respectively, making it competent to immobilize mass anti-ANXA2 antibodies. The nano-interface platform is fabricated through electrophoretic deposition of HsGDY onto indium tin oxide (ITO) coated glass substrate (50V, 60s) with subsequent immobilization of anti-ANXA2 biomolecules and bovine serum albumin (BSA) to minimize non-specific binding. The pristine HsGDY and fabricated electrodes were characterized using spectroscopic, microscopic, zetasizer, surface area and pore size analyzer as well as electrochemical techniques. The electrochemical response of fabricated HsGDY nano-interface based biosensing platform (BSA/anti-ANXA2/HsGDY/ITO) is investigated via cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques, which covers a wider linear detection range in between 0.01 fg mL^-1 to 1000 ng mL^-1 along with an exceptional sensitivity of 13.8 μA [log (ng mL^-1)]^-1 cm^-2 and 2.8 μA [log (ng mL^-1)]^-1 cm^-2 via CV and DPV techniques, respectively. This developed biosensor has the ability for unprecedented ultralow level i.e., upto 3 molecules of ANXA2 cancer biomarker detection. Moreover, the obtained electrochemical results show excellent correlation with the concentration of ANXA2 cancer biomarker present in LC patients obtained through enzyme linked immunosorbent assay (ELISA) technique.

Rational design of smart nano-platforms based on antifouling-nanomaterials toward multifunctional bioanalysis

The ability to design nanoprobe devices with the capability of quantitative/qualitative operation in complex media will probably underpin the main upcoming progress in healthcare research and development. However, the biomolecules abundances in real samples can considerably alter the interface performance, where unwanted adsorption/adhesion can block signal response and significantly decrease the specificity of the assay. Herein, this review firstly offers a brief outline of several significances of fabricating high-sensitivity and low-background interfaces to adjust various targets' behaviors induced via bioactive molecules on the surface. Besides, some important strategies to resist non-specific protein adsorption and cell adhesion, followed by imperative categories of antifouling reagents utilized in the construction of high-performance solid sensory interfaces, are discussed. The next section specifically highlights the various nanocomposite probes based on antifouling-nanomaterials for electrode modification containing carbon nanomaterials, noble metal nanoparticles, magnetic nanoparticles, polymer, and silicon-based materials in terms of nanoparticles, rods, or porous materials through optical or chemical strategies. We specially outline those nanoprobes that are capable of identification in complex media or those using new constructions/methods. Finally, the necessity and requirements for future advances in this emerging field are also presented, followed by opportunities and challenges.

A simple label-free electrochemical sensor for sensitive detection of alpha-fetoprotein based on specific aptamer immobilized platinum nanoparticles/carboxylated-graphene oxide

A label-free electrochemical aptamer-based sensor has been fabricated for alpha-fetoprotein (AFP) detection. Platinum nanoparticles on carboxylated-graphene oxide (PtNPs/GO-COOH) modified screen-printed graphene-carbon paste electrode (SPGE) was utilized as an immobilization platform, and the AFP aptamer was employed as a bio-recognition element. The synthesized GO-COOH helps to increase the surface area and amounts of the immobilized aptamer. Subsequently, PtNPs are decorated on GO-COOH to enhance electrical conductivity and an oxidation current of the hydroquinone electrochemical probe. The aptamer selectively interacts with AFP, causing a decrease in the peak current of the hydroquinone because the binding biomolecules on the electrode surface hinder the electron transfer of the redox probe. Effects of aptamer concentration and AFP incubation time were studied, and the current changes of the redox probe before and after AFP binding were investigated by square wave voltammetry. The developed aptasensor provides a linear range from 3.0–30 ng mL⁻¹ with a detection limit of 1.22 ng mL⁻¹. Moreover, the aptamer immobilized electrode offers high selectivity to AFP molecules, good stability, and sensitive determination of AFP in human serum samples with high recoveries.

The impact of antifouling layers in fabricating bioactive surfaces

Bioactive surfaces modified with functional peptides are critical for both fundamental research and practical application of implant materials and tissue repair. However, when bioactive molecules are tethered on biomaterial surfaces, their functions can be compromised due to unwanted fouling (mainly nonspecific protein adsorption and cell adhesion). In recent years, researchers have continuously studied antifouling strategies to obtain low background noise and effectively present the function of bioactive molecules. In this review, we describe several commonly used antifouling strategies and analyzed their advantages and drawbacks. Among these strategies, antifouling molecules are widely used to construct the antifouling layer of various bioactive surfaces. Subsequently, we summarize various structures of antifouling molecules and their surface grafting methods and characteristics. Application of these functionalized surfaces in microarray, biosensors, and implants are also introduced. Finally, we discuss the primary challenges associated with antifouling layers in fabricating bioactive surfaces and provide prospects for the future development of this field. STATEMENT OF SIGNIFICANCE: The nonspecific protein adsorption and cell adhesion will cause unwanted background "noise" on the surface of biological materials and detecting devices and compromise the performance of functional molecules and, therefore, impair the performance of materials and the sensitivity of devices. In addition, the selection of antifouling surfaces with proper chain length and high grafting density is also of great importance and requires further studies. Otherwise, the surface-tethered bioactive molecules may not function in their optimal status or even fail to display their functions. Based on these two critical issues, we summarize antifouling molecules with different structures, variable grafting methods, and diverse applications in biomaterials and biomedical devices reported in literature. Overall, we expect to shed some light on choosing the appropriate antifouling molecules in fabricating bioactive surfaces.

An aptasensor using ceria electrodeposited-screen-printed carbon electrode for detection of epithelial sodium channel protein as a hypertension biomarker

Epithelial sodium channel (ENaC) is a transmembrane protein that has an essential role in maintaining the levels of sodium in blood plasma. A person with a family history of hypertension has a high enough amount of ENaC protein in the kidneys or other organs, so that the ENaC protein acts as a marker that a person is susceptible to hypertension. An aptasensor involves aptamers, which are oligonucleotides that function similar to antibodies, as sensing elements. An electrochemical aptasensor for the detection of ENaC was developed using a screen-printed carbon electrode (SPCE) which was modified by electrodeposition of cerium oxide (CeO2). The aptamer immobilization was via the streptavidin-biotin system. The measurement of changes in current of the active redox [Fe(CN)6]3-/4- was carried out by differential pulse voltammetry. The surfaces of SPCE and SPCE/CeO2 were characterized using scanning electron microscopy, voltammetry and electrochemical impedance spectroscopy. The Box-Behnken experimental optimization design revealed the streptavidin incubation time, aptamer incubation time and streptavidin concentrations were 30 min, 30 min and 10.8 µg ml-1, respectively. Various concentrations of ENaC were used to obtain the linearity range of 0.05-3.0 ng ml-1, and the limits of detection and quantification were 0.012 ng ml-1 and 0.038 ng ml-1, respectively. This aptasensor method has the potential to measure the ENaC protein levels in urine samples as well as to be a point-of-care device.

Highly selective and antifouling electrochemical biosensors for sensitive MicroRNA assaying based on conducting polymer polyaniline functionalized with zwitterionic peptide

Ultrasensitive and low-fouling microRNA electrochemical biosensors were successfully constructed by introducing thiol-terminated antifouling molecules (peptide sequence, polyethylene glycol, or mercapto alcohol) onto the surface of polyaniline-modified electrodes. For the three kinds of antifouling materials investigated, the newly designed and synthesized peptide exhibited superior antifouling ability to others, and it could effectively reduce the nonspecific adsorption of proteins and even prevent the fouling effect of serum. Compared with microRNA biosensors without antifouling capability, or those modified with polyethylene glycol or mercapto alcohol, the biosensor modified with the designed zwitterionic peptide showed the highest specificity for single-base mismatch, three-base mismatch, and completely complementary microRNAs. Most interestingly, the experimental results indicated that the introduction of antifouling molecules to the sensing interfaces did not significantly change the sensitivity of the biosensor. The strategy of constructing antifouling biosensors based on newly synthesized zwitterionic peptides and conducting polymers can be promisingly extended to the development of other electrochemical sensors and biosensors without encountering biofouling. Graphical abstract Ultrasensitive and low-fouling microRNA electrochemical biosensors were constructed by introducing thiol-terminated antifouling molecules (peptide sequence, polyethylene glycol, or mercapto alcohol) onto the surface of polyaniline-modified electrodes. The biosensor modified with the designed zwitterionic peptide showed the highest specificity amongst four kinds of biosensors.

Polymeric immunosensors for tumor detection

Cancer is a broad-spectrum disease which is spread globally, having high mortality rates. This results from genetic, epigenetic and molecular abnormalities caused by various mutations. The main reason behind this critical problem lies in its diagnostics, the late detection of the disease is the root cause of all this. This can be managed well by the timely diagnosis of cancer by means of the tumor biomarkers present in the body fluids such as serum, blood, and urine. These tumor biomarkers are present in normal conditions as well, but their concentrations are altered in the presence of a malignant tumor. Prolonged studies have reported that immunosensors can be used to detect the minimal amount of biomarkers present in the sample and also provides point-of-care detection. The recent investigations demonstrated the use of polymers along with immunosensors for enhancing their selectivity and sensitivity towards the biomarkers and making them even more efficient. This review focuses on the variety of tumor biomarkers, different types of immunosensors and polymeric immunosensors using different polymers like polypyrrole, polyaniline, PHEMA, etc.

Nanostructure-based Sensitive Electrochemical Immunosensors

It is well-known that electrochemical immunosensors have many advantages, including but not limited to high sensitivity, simplicity in application, low-cost production, automated control and potential miniaturization. Due to specific antigen–antibody recognition, electrochemical immunosensors also have provided exceptional possibilities for real-time trace detection of analytical biotargets, which consists of small molecules (such as natural toxins and haptens), macromolecules, cells, bacteria, pathogens or viruses. Recently, the advances in the development of electrochemical immunosensors can be classified into the following directions: the first is using electrochemical detection techniques (voltammetric, amperometric, impedance spectroscopic, potentiometric, piezoelectric, conductometric and alternating current voltammetric) to achieve high sensitivity regarding the electrochemical change of electrochemical signal transduction; the second direction is developing sensor configurations (microfluidic and paper-based platforms, microelectrodes and electrode arrays) for simultaneous multiplex high-throughput analyses; and the last is designing nanostructured materials serving as sensing interfaces to improve sensor sensitivity and selectivity. This chapter introduces the working principle and summarizes the state-of-the-art of electrochemical immunosensors during the past few years with practically relevant details for: (a) metal nanoparticle- and quantum dot-labeled immunosensors; (b) enzyme-labeled immunosensors; and (c) magnetoimmunosensors. The importance of various types of nanomaterials is also thoroughly reviewed to obtain an insight into understanding the theoretical basis and practical orientation for the next generation of diagnostic devices.

Electrochemical sensors and biosensors based on the use of polyaniline and its nanocomposites: a review on recent advances

Polyaniline and its composites with nanoparticles have been widely used in electrochemical sensor and biosensors due to their attractive properties and the option of tuning them by proper choice of materials. The review (with 191 references) describes the progress made in the recent years in polyaniline-based biosensors and their applications in clinical sensing, food quality control, and environmental monitoring. A first section summarizes the features of using polyaniline in biosensing systems. A subsequent section covers sensors for clinical applications (with subsections on the detection of cancer cells and bacteria, and sensing of glucose, uric acid, and cholesterol). Further sections discuss sensors for use in the food industry (such as for sulfite, phenolic compounds, acrylamide), and in environmental monitoring (mainly pesticides and heavy metal ions). A concluding section summarizes the current state, highlights some of the challenges currently compromising performance in biosensors and nanobiosensors, and discusses potential future directions. Graphical abstract Schematic presentation of electrochemical sensor and biosensors applications based on polyaniline/nanoparticles in various fields of human life including medicine, food industry, and environmental monitoring. The simultaneous use of suitable properties polyaniline and nanoparticles can provide the fabrication of sensing systems with high sensitivity, short response time, high signal/noise ratio, low detection limit, and wide linear range by improving conductivity and the large surface area for biomolecules immobilization.

Electrochemical Aptasensor for Ultralow Fouling Cancer Cell Quantification in Complex Biological Media Based on Designed Branched Peptides

The rapid, convenient, and selective assaying of clinical targets directly in complex biological media brings with it the potential to revolutionize diagnostics. One major hurdle to impact is retention of selectivity and a tight control of nonspecific surface interactions or biofouling. We report herein, the construction of an antifouling interface through the covalent attachment of designed branched zwitterionic peptides onto electrodeposited polyaniline film. The antifouling capability of the designed branched peptide significantly outperforms that of the commonly used PEG and linear peptides. The interfaces modified with branched peptides are exceptionally effective in reducing a nonspecific protein and cell adsorption, as verified by electrochemical and fluorescent characterization. The derived sensors with mucin1 protein (MUC1) aptamer as the recognition element detect MUC1-positive MCF-7 breast cancer cells in human serum with high sensitivity and selectivity. The linear response range of the cytosensor for the MCF-7 cell is from 50 to 10⁶ cells/mL, with a limit of detection as low as 20 cells/mL. More importantly, the assaying performances remain unchanged in human serum owing to the presence of branched antifouling peptide, indicating feasibility of the cytosensor for practical cancer cell quantification in complex samples.

A novel electrochemical immunosensor based on Au nanoparticles and horseradish peroxidase signal amplification for ultrasensitive detection of α-fetoprotein

An electrochemical double-layer Au nanoparticle membrane immunosensor was developed using an electrochemical biosensing signal amplification system with Au nanoparticles, thionine, chitosan, and horseradish peroxidase, which was fabricated using double self-adsorption of Au nanoparticle sol followed by anti-α-fetoprotein Balb/c mouse monoclonal antibody adsorption. The AuNPs sol was characterized by spectrum scanning and transmission electron microscopy. The immunosensor was characterized by atomic force microscopy, cyclic voltammetry, and alternating-current impedance during each stage of adsorption and assembly. The amperometric I-t curve method was used to measure α-fetoprotein (AFP) diluted in phosphate buffered saline. The result indicated a wide linear range, and the change rate of steady-current before and after immune response had linear correlation within the range 0.1-10⁴ pg/mL AFP. The current change rate equation was △I = 5.82334 lgC + 37.01195 (R² = 0.9922). The lowest limit of detection was 0.03 pg/mL (S/N = 3), and the reproducibility of the sensor was good. Additionally, the sensor could be stably stored above phosphate buffered saline at 4 °C for more than 24 days. More importantly, the sensor is label-free, reagentless and low fouling, making it capable of assaying AFP in real serum samples without suffering from significant interference or biofouling.

Label-free electrochemical aptasensor for detection of alpha-fetoprotein based on AFP-aptamer and thionin/reduced graphene oxide/gold nanoparticles

Sensitive and accurate detection of tumor markers is critical to early diagnosis, point-of-care and portable medical supervision. Alpha fetoprotein (AFP) is an important clinical tumor marker for hepatocellular carcinoma (HCC), and the concentration of AFP in human serum is related to the stage of HCC. In this paper, a label-free electrochemical aptasensor for AFP detection was fabricated using AFP-aptamer as the recognition molecule and thionin/reduced graphene oxide/gold nanoparticles (TH/RGO/Au NPs) as the sensor platform. With high electrocatalytic property and large specific surface area, RGO and Au NPs were employed on the screen-printed carbon electrode to load TH molecules. The TH not only acted as a bridging molecule to effectively capture and immobilize AFP-aptamer, but as the electron transfer mediator to provide the electrochemical signal. The AFP detection was based on the monitoring of the electrochemical current response change of TH by the differential pulse voltammetry. Under optimal conditions, the electrochemical responses were proportional to the AFP concentration in the range of 0.1-100.0 μg/mL. The limit of detection was 0.050 μg/mL at a signal-to-noise ratio of 3. The proposed method may provide a promising application of aptamer with the properties of facile procedure, low cost, high selectivity in clinic.

Current Technologies of Electrochemical Immunosensors: Perspective on Signal Amplification

An electrochemical immunosensor employs antibodies as capture and detection means to produce electrical charges for the quantitative analysis of target molecules. This sensor type can be utilized as a miniaturized device for the detection of point-of-care testing (POCT). Achieving high-performance analysis regarding sensitivity has been one of the key issues with developing this type of biosensor system. Many modern nanotechnology efforts allowed for the development of innovative electrochemical biosensors with high sensitivity by employing various nanomaterials that facilitate the electron transfer and carrying capacity of signal tracers in combination with surface modification and bioconjugation techniques. In this review, we introduce novel nanomaterials (e.g., carbon nanotube, graphene, indium tin oxide, nanowire and metallic nanoparticles) in order to construct a high-performance electrode. Also, we describe how to increase the number of signal tracers by employing nanomaterials as carriers and making the polymeric enzyme complex associated with redox cycling for signal amplification. The pros and cons of each method are considered throughout this review. We expect that these reviewed strategies for signal enhancement will be applied to the next versions of lateral-flow paper chromatography and microfluidic immunosensor, which are considered the most practical POCT biosensor platforms.

Nanomaterial-doped conducting polymers for electrochemical sensors and biosensors

This review summarizes recent advances in the development of electrochemical sensors and biosensors based on nanomaterial doped conducting polymers.

A Fe3O4@Au-basedpseudo-homogeneous electrochemical immunosensor for AFP measurement using AFP antibody-GNPs-HRP as detection probe

In this study, a Fe₃O₄@Au-based pseudo-homogeneous electrochemical immunosensor was prepared for detection of alpha fetoprotein (AFP), a well-known hepatocellular carcinoma biomarker. The primary antibody (Ab1) was immobilized on Fe₃O₄@Au NPs as the capture probe. Horseradish peroxidase (HRP) and secondary antibody (Ab2) were conjugated on gold nanoparticles (GNPs) through electrostatic adsorption to form signal-amplifying labels. In the presence of AFP, a sandwich immunocomplex was formed via specific recognition of antigen-antibody in a Fe₃O₄@Au-basedpseudo-homogeneousreaction system. After the immunocomplex was captured to the surface of magnetic glassy carbon electrode (MGCE), the labeling HRP catalyzed the decomposition of H₂O₂, resulting in a substantial current for the quantitative detection of AFP. The amperometric (i-t) method was employed to record the response signal of the immunosensor based on the catalysis of the immobilized HRP toward the reduction of H₂O₂ with hydroquinone (HQ) as the redox mediator. Under the optimal conditions, the amperometric current response presented a linear relationship with AFP concentration over the range of 20 ng/mL-100 ng/mLwith a correlation coefficient of 0.9940, and the detection limit was 0.64 ng/mL at signal/noise [S/N] = 3. Moreover, the electrochemical immunosensor exhibited higher anti-interference ability, acceptable reproducibility and long-term stability for AFP detection.

Protein adsorption on implant metals with various deformed surfaces

The adsorption of proteins has great influence on the biocompatibility, lubrication and corrosion properties of implantable metals. The subsurface microstructure of metals can be easily changed during the manufacturing or service processes and it is easily ignored. The same chemical composition but with various surface microstructures can result in very different adsorption behaviour. In this study, mechanical polishing, electrochemical polishing and shot peening methods were used to generate different gradient deformed surfaces for CoCrMo alloys. The increase of the residual compressive stress and the decrease of grains to a nano-size on the deformed subsurface can effectively enhance the activity of metal atoms and improve the element diffusion, that is helpful in forming a more compact passive film. This can increase the contents of oxides on the surface, and then enhance the electrostatic force and increase the surface's positive charge density as well as adsorption sites. All of the above facilitated the adsorption of the negatively charged albumin onto the alloy surface.

A sandwich-type electrochemical biosensor for alpha-fetoprotein based on Au nanoparticles decorating a hollow molybdenum disulfide microbox coupled with a hybridization chain reaction

In this work, a sensitive sandwich-type biosensor for detecting alpha-fetoprotein (AFP) is developed by using a target-triggered hybridization chain reaction strategy.